.TH std::shift_left,std::shift_right 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::shift_left,std::shift_right \- std::shift_left,std::shift_right

.SH Synopsis
   Defined in header <algorithm>
   template< class ForwardIt >

   constexpr ForwardIt shift_left( ForwardIt first, ForwardIt last,
                                   typename                           \fB(1)\fP \fI(since C++20)\fP
   std::iterator_traits<ForwardIt>::

                                       difference_type n );
   template< class ExecutionPolicy, class ForwardIt >

   ForwardIt shift_left( ExecutionPolicy&& policy,
                         ForwardIt first, ForwardIt last,             \fB(2)\fP \fI(since C++20)\fP
                         typename std::iterator_traits<ForwardIt>::

                             difference_type n );
   template< class ForwardIt >

   constexpr ForwardIt shift_right( ForwardIt first, ForwardIt last,
                                    typename                          \fB(3)\fP \fI(since C++20)\fP
   std::iterator_traits<ForwardIt>::

                                        difference_type n );
   template< class ExecutionPolicy, class ForwardIt >

   ForwardIt shift_right( ExecutionPolicy&& policy,
                          ForwardIt first, ForwardIt last,            \fB(4)\fP \fI(since C++20)\fP
                          typename std::iterator_traits<ForwardIt>::

                              difference_type n );

   Shifts the elements in the range [first, last) by n positions.

   1) Shifts the elements towards the beginning of the range.
     * If n == 0 || n >= last - first, there are no effects.
     * Otherwise, for every integer i in [0, last - first - n), moves the element
       originally at position first + n + i to position first + i.
   The moves are performed in increasing order of i starting from 0.
   3) Shifts the elements towards the end of the range.
     * If n == 0 || n >= last - first, there are no effects.
     * Otherwise, for every integer i in [0, last - first - n), moves the element
       originally at position first + i to position first + n + i.
   If ForwardIt meets the LegacyBidirectionalIterator requirements, then the moves are
   performed in decreasing order of i starting from last - first - n - 1.
   2,4) Same as \fB(1)\fP and \fB(3)\fP, respectively, but executed according to policy and the
   moves may be performed in any order.
   These overloads participate in overload resolution only if
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

   Elements that are in the original range but not the new range are left in a valid
   but unspecified state.

   If any of the following conditions is satisfied, the behavior is undefined:

     * n >= 0 is not true.
     * The type of *first is not MoveAssignable.
     * For shift_right, ForwardIt is neither LegacyBidirectionalIterator nor
       ValueSwappable.

.SH Parameters

   first   -  the beginning of the original range
   last    -  the end of the original range
   n       -  the number of positions to shift
   policy  -  the execution policy to use. See execution policy for details.
.SH Type requirements
   -
   ForwardIt must meet the requirements of LegacyForwardIterator.

.SH Return value

   1,2) The end of the resulting range.
     * If n is less than std::distance(first, last), returns an iterator equal to
       std::next(first, (std::distance(first, last) - n)).
     * Otherwise, returns first.
   3,4) The beginning of the resulting range.
     * If n is less than std::distance(first, last), returns an iterator equal to
       std::next(first, n).
     * Otherwise, returns last.

.SH Complexity

   1,2) At most std::distance(first, last) - n assignments.
   3,4) At most std::distance(first, last) - n assignment or swaps.

.SH Exceptions

   The overloads with a template parameter named ExecutionPolicy report errors as
   follows:

     * If execution of a function invoked as part of the algorithm throws an exception
       and ExecutionPolicy is one of the standard policies, std::terminate is called.
       For any other ExecutionPolicy, the behavior is implementation-defined.
     * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

.SH Notes

   Feature-test macro  Value    Std                 Feature
   __cpp_lib_shift    201806L (C++20) std::shift_left and std::shift_right

.SH Example


// Run this code

 #include <algorithm>
 #include <iostream>
 #include <string>
 #include <type_traits>
 #include <vector>

 struct S
 {
     int value{0};
     bool specified_state{true};

     S(int v = 0) : value{v} {}
     S(S const& rhs) = default;
     S(S&& rhs) { *this = std::move(rhs); }
     S& operator=(S const& rhs) = default;
     S& operator=(S&& rhs)
     {
         if (this != &rhs)
         {
             value = rhs.value;
             specified_state = rhs.specified_state;
             rhs.specified_state = false;
         }
         return *this;
     }
 };

 template<typename T>
 std::ostream& operator<<(std::ostream& os, std::vector<T> const& v)
 {
     for (const auto& s : v)
     {
         if constexpr (std::is_same_v<T, S>)
             s.specified_state ? os << s.value << ' ' : os << ". ";
         else if constexpr (std::is_same_v<T, std::string>)
             os << (s.empty() ? "." : s) << ' ';
         else
             os << s << ' ';
     }
     return os;
 }

 int main()
 {
     std::cout << std::left;

     std::vector<S>           a{1, 2, 3, 4, 5, 6, 7};
     std::vector<int>         b{1, 2, 3, 4, 5, 6, 7};
     std::vector<std::string> c{"α", "β", "γ", "δ", "ε", "ζ", "η"};

     std::cout << "vector<S> \\tvector<int> \\tvector<string>\\n";
     std::cout << a << "  " << b << "  " << c << '\\n';

     std::shift_left(begin(a), end(a), 3);
     std::shift_left(begin(b), end(b), 3);
     std::shift_left(begin(c), end(c), 3);
     std::cout << a << "  " << b << "  " << c << '\\n';

     std::shift_right(begin(a), end(a), 2);
     std::shift_right(begin(b), end(b), 2);
     std::shift_right(begin(c), end(c), 2);
     std::cout << a << "  " << b << "  " << c << '\\n';

     std::shift_left(begin(a), end(a), 8); // has no effect: n >= last - first
     std::shift_left(begin(b), end(b), 8); // ditto
     std::shift_left(begin(c), end(c), 8); // ditto
     std::cout << a << "  " << b << "  " << c << '\\n';

 //  std::shift_left(begin(a), end(a), -3); // UB, e.g. segfault
 }

.SH Possible output:

 vector<S>       vector<int>     vector<string>
 1 2 3 4 5 6 7   1 2 3 4 5 6 7   α β γ δ ε ζ η
 4 5 6 7 . . .   4 5 6 7 5 6 7   δ ε ζ η . . .
 . . 4 5 6 7 .   4 5 4 5 6 7 5   . . δ ε ζ η .
 . . 4 5 6 7 .   4 5 4 5 6 7 5   . . δ ε ζ η .

.SH See also

   move                moves a range of elements to a new location
   \fI(C++11)\fP             \fI(function template)\fP
   move_backward       moves a range of elements to a new location in backwards order
   \fI(C++11)\fP             \fI(function template)\fP
   rotate              rotates the order of elements in a range
                       \fI(function template)\fP
   ranges::shift_left  shifts elements in a range
   ranges::shift_right (niebloid)
   (C++23)
